186 OUTLINES OF EVOLUTIONARY BIOLOGY 



the developing organism, but a character of far more deeply 

 seated origin. 



It must not be forgotten that the chromosomes, as such, are 

 only recognizable during the process of mitosis ; in the resting 

 condition of the nucleus they appear to be broken up into larger 

 or smaller granules of chromatin scattered through the linin 

 reticulum. The observations on accessory chromosomes above 

 mentioned, and others to be referred to presently, point to the 



6 (Unreduced Cells) $( Unreduced Cells) 



Reduction 



(4+1) 4 



5 Spermatozoa 



(4 + 1) 



10(9 Zygote) 



9(6 Zygote) 



FIG. 67. Diagram illustrating the Correlation between the Number of 

 Chromosomes and the Sex in certain Insects. (The numbers of chromo- 

 somes given in this diagram are arbitrarily chosen and are obviously 

 different from those which occur in Stenobothi us, as shown in Fig. 66.) 



conclusion, however, that, in spite of this, the different chromo- 

 somes preserve some sort of individuality from one cell-genera- 

 tion to another. In other words, we have reason to believe that 

 the chromosomes which make their appearance at the onset of 

 each mitosis are, taken each as a whole, the same as those which 

 become disintegrated at the close of the preceding mitosis, though 

 it is very possible that the constituent parts of the old chromo- 

 somes (chromatin granules, chromomeres or ids), after absorbing 

 nutriment and increasing in size during the resting period, may 

 come together in new combinations to form the new chromosomes 

 each time division of the nucleus takes place. 1 



1 Vide Farmer, Croonian Lecture, Proc. Itoyal Soc., Ser. B, Vol. 79, 1907. 



